1. Field of the Invention
This invention relates to an electrophotographic photosensitive member, and a process cartridge and an electrophotographic apparatus which have the electrophotographic photosensitive member.
2. Related Background Art
Electrophotographic photosensitive members are repeatedly put to means for charging, exposure, development, transfer, cleaning and charge elimination. An electrostatic latent image formed upon charging and exposure is made into a toner image by the use of a fine-particle developer called a toner. This toner image is further transferred to a transfer medium such as paper by a transfer means, where the toner of the toner image is not all transferred, but partly remains on the surface of the photosensitive member.
The remaining toner (residual toner) is removed by a cleaner, or, on account of the advancement of cleanerless techniques in recent years, the residual toner is collected by what is called a cleaning-at-development system in which any independent cleaning means is not provided and the residual toner is collected through a developing means.
Electrophotographic photosensitive members, to which electrical and mechanical external forces as stated above are directly applied, are also required to have durability to such forces. Stated specifically, they are required to have durability to the occurrence of surface wear and scratches due to friction and durability to the deterioration of surface layer that is caused by adhesion of active substances such as ozone and NOx generated at the time of charging.
To meet such requirements imposed on electrophotographic photosensitive members, it has been attempted to provide protective layers of various types. In particular, protective layers composed chiefly of resins have been proposed in a large number. For example, as disclosed in Japanese Patent Application Laid-open No. 57-30846, a protective layer is proposed which is formed of a resin to which a metal oxide is added as conductive particles so that its resistance can be controlled.
Such conductive particles are dispersed in the protective layer of an electrophotographic photosensitive member chiefly in order to control the electrical resistance of the protective layer itself to prevent residual potential from increasing in the photosensitive member as the electrophotographic process is repeatedly used. It is known that suitable resistance values of protective layers for electrophotographic photosensitive members are 1010 to 1015 xcexa9xc2x7cm. In respect of abrasion wear due to repeated used, it is advantageous for the mass ratio of the mass (P) of conductive particles to the mass (B) of binder resin, P/B, to be smaller, i.e., for the binder resin to be in a larger quantity than the conductive particles.
Meanwhile, in a protective layer containing a charge-transporting material, the mass ratio of the mass (D) of charge-transporting material to the mass (B) of binder resin, D/B, is about 2/1 to 1/2, in order for the layer to have a low residual potential. In general, its residual potential can be made smaller by making the value of D/B larger, but this may cause a great abrasion for the film of a protective layer, or, when a curable resin is used, the curing of the curable resin may be inhibited.
As stated above, in recent years studies are being made on how to improve the performance of electrophotographic photosensitive members with resort to protective layers. However, compared with the thickness of usual photosensitive layers which is tens of xcexcm, the thickness of protective layers is usually as small as a few xcexcm. Thus, in order to maintain the like durability, it is, of course, necessary for the protective layers to be more kept from being scratched and abraded. Accordingly, studies are being made on protective layers the resin of which is replaced with a curable resin, and efforts are made on how to make the layer harder and less abrasive. However, with progress of studies actually made taking note of only the hardness, it has been realized that although the layer is hard, it tends to be scratched to have a poor durability after all, or although it is not so hard, it is well balanced with abrasion wear to bring about an improvement in durability in total.
Too low hardness also makes the abrasion wear worse as a matter of course. Especially when continuing to use a layer having not so high hardness in spite of the use of a curable resin, black dots may occur if a reverse development system is used. Such black dots differ from black dots having ever come into question, and are caused neither by simple injection of holes from the support nor by generation of holes due to heat or electric field generated from a charge generation layer even at the initial stage. This has become apparent as a result of studies made by the present inventors. The real cause of such black dots has not been regretfully elucidated, but it has been realized at least that the black dots occur after extensive operation on thousands to tens of thousands of sheets when a photosensitive member is used which has the photosensitive layer and the protective layer on a conductive support and also that they occur when the protective layer has a specific hardness.
An object of the present invention is to provide an electrophotographic photosensitive member which has a surface layer free of cracks and having a superior durability to the occurrence of surface wear and scratches, does not cause black dots upon running (or extensive operation) which are inherent in the electrophotographic photosensitive member having the above protective layer, and can maintain a high-grade image quality; and also to provide a process cartridge and an electrophotographic apparatus which have such an electrophotographic photosensitive member.
To achieve the above object, the present invention provides an electrophotographic photosensitive member comprising a support, and a photosensitive layer and a protective layer which have been formed on the support in this order;
a thickness d (xcexcm) of the protective layer, a universal hardness Hu-1 (N/mm2) of the protective layer, and a universal hardness Hu-2 (N/mm2) of the photosensitive layer as measured after the protective layer is peeled off satisfying the following expression (1):
5.8xc3x97d+Hu-2xe2x89xa6Hu-1xe2x89xa6xe2x88x922.45xc3x97d2+44.4xc3x97d+Hu-2xe2x80x83xe2x80x83(1). 
The present invention also provides a process cartridge comprising an electrophotographic photosensitive member and at least one means selected from the group consisting of a charging means, a developing means and a cleaning means;
the electrophotographic photosensitive member and at least one means being supported as one unit and being detachably mountable on the main body of an electrophotographic apparatus; and
the electrophotographic photosensitive member comprising a support, and a photosensitive layer and a protective layer which have been formed on the support in this order;
a thickness d (xcexcm) of the protective layer, a universal hardness Hu-1 (N/mm2) of the protective layer, and a universal hardness Hu-2 (N/mm2) of the photosensitive layer as measured after the protective layer is peeled off satisfying the following expression (1):
5.8xc3x97d+Hu-2xe2x89xa6Hu-1xe2x89xa6xe2x88x922.45xc3x97d2+44.4xc3x97d+Hu-2xe2x80x83xe2x80x83(1). 
The present invention still also provides an electrophotographic apparatus comprising an electrophotographic photosensitive member, a charging means, an exposure means, a developing means and a transfer means;
the electrophotographic photosensitive member comprising a support, and a photosensitive layer and a protective layer which have been formed on the support in this order;
a thickness d (xcexcm) of the protective layer, a universal hardness Hu-1 (N/mm2) of the protective layer, and a universal hardness Hu-2 (N/mm2) of the photosensitive layer as measured after the protective layer is peeled off satisfying the following expression (1):
5.8xc3x97d+Hu-2xe2x89xa6Hu-1xe2x89xa6xe2x88x922.45xc3x97d2+44.4xc3x97d+Hu-2xe2x80x83xe2x80x83(1). 